OP, I think everyone will feel nervous no matter their age. If they are 22 they will worry they are too young, if they are 32 they will worry they are too old. People with a BSc will worry they don’t have Masters or PhD or other degrees, people with more degrees and life experience will worry they didn’t take a more direct route.
Everyone who is lucky enough to get an offer has something special about them that the medical school saw. No matter your background you have something great to bring to the table. Focus on that. You are enough the way you are. I know I’m so excited to meet my classmates, no matter how old they are. I think most people in your class will be excited to meet you too.

I agree it is more a measure of who is keen more than anything else. However, the checklist states clearly that you should not register for classes unless you are very sure that you are going to stick with this offer.
While plenty of people will hold the spot with the $500, I doubt that anyone would go to the trouble of registering for courses unless they are completely sure.
So that number is probably a measure of how many people are both keen and conscientious about completing their checklists and sure they want to accept Dal :)

Your most recent MCAT score is good and you have been getting interviews at U of M, which means that you are not being eliminated by your GPA or MCAT score. I think it’s possible that if you focus on your interviewing skills that you could be offered acceptance in the next cycle.
In the meantime, I would focus on improving your overall application. You could finish the 4 year degree and get A+ in the courses you have left, increasing your GPA. You could get a job in research or start a masters degree to improve your research experience. You could pick up a couple of interesting ECs, possibly shadow some physicians in your area.
Are you interested in practicing medicine in Canada? I wouldn’t apply to US schools or Ireland unless you are interested in working in those areas. It’s becoming more and more difficult to return to Canada for residency after doing an MD in the US or abroad. It’s also so much more expensive.
U of M has some of the lowest medical degree tuition fees you’ll ever see. I would take a year or two and improve your application and reapply. Practice, practice, practice for your MMI. Gain some life experiences that make you a more interesting and competitive candidate. If you want to apply to a few additional Canadian schools like Queens, McMaster, or UBC, you could do that, but honestly you have such an advantage at U of M.

I definitely think you should apply.
That’s wonderful that your CARS score is high. For the bio/biochem and chem/phys score, keep studying and practicing as much as possible between now and August to improve your score. The Khan Academy was pretty good review for those topics and it’s free. r/MCAT also has a great deal of advice on improving your scores in those sections. Perhaps consider a course or buying the books from one of the MCAT courses?
I would definitely apply if I were you. Toronto also has an indigenous student application program and with their grade weighting you may qualify for their GPA cutoff. I would call U of T’s Office of Indigenous Medical Education and talk to them. It’s worthwhile calling the admissions committee at McMaster and Queens as well to ask for clarification if you have further questions after reviewing their websites. They are all very helpful.
Best of luck!

Some of the best doctors and medical students that I know started around that age, after pursing other degrees and careers. I think your life experiences and your PhD confer an advantage, if anything.
Everyone who is applying to medicine should do a careful cost/benefit analysis before committing to an MD. You may have family considerations, for example. However if it’s only the stigma of being a little bit older, then I wouldn’t let that hold you back at all.

Yes, RIM is a mandatory part of the MD curriculum. How large and elaborate your research project is will vary from student to student. For example, if a student doesn't have research experience before medical school and felt a bit intimidated, the school would help them design a smaller project that isn't too arduous. But you can do larger projects too, if that's your interest.
If you wanted to apply for Tri-Council Agency (NSERC, CIHR, SSHRC) funding to help with costs, I'm sure potential supervisors would be even more open to taking you on as a summer student. Undergraduate students are typically eligible for a studentship following second year (Summer 2020 for you). The application process requires a little bit of planning with potential supervisors. The deadline is typically some time in January, so maybe you could reach out to potential supervisors this autumn. Here is a link with a bit of information for the Dalhousie Application this year:
https://cdn.dal.ca/content/dam/dalhousie/images/faculty/science/2019 Summer Research Awards - Guidelines.pdf
If you are interested in a basic science project, you would apply to NSERC, and if you were thinking of a project with more of a clinical focus (working with patients or patient samples) you would apply to CIHR.
If successful, the award would help fund your studentship. It also looks really good on your CV and it's a good practice for applying to funding/grants.

Since you're early in your undergraduate career, would you consider applying to do a summer research internship at Dalhousie University from May to August? That way you would get to know Halifax for 4 months in the summer and get a better idea of if would be a good fit for you in the future. It would also allow you to make connections with the faculty members there and help you gain research experience. Research is a big component of the Dalhousie Med program -- every student does a research project through the Research In Medicine component of the curriculum
The maritime connection is really important to get an interview for Dalhousie Medicine. Excellent GPA is important, but not sufficient on its own. You will have to identify and clearly articulate what you really like about the Maritimes and what has inspired you to apply to Dalhousie.

This!
Honestly, I think you have a fair shot at Queens, U of T, and Dalhousie. I had a friend who interviewed at Queens this year who also had a PhD and a similar GPA as you. For McMaster, you will need a very strong CARS. CASPer is also 1/3 of your pre-interview score for McMaster, so if you can practice a good deal for that, that would help your chances too.
Your accolades are really impressive, you should be proud of yourself, and we need more people like you in medical school! I think you should go for it. I wouldn't do a second undergrad.

If you wanted to apply to be a part-time lecturer or teaching assistant, the best place to start is probably is to check the job postings:
https://3912.cupe.ca/job-postings/
You may notice that many of the postings at Dalhousie require a PhD in the discipline that is being taught. The minimum requirement is usually a graduate degree in that discipline, with PhD preferred. I know for lower year courses (with large class sizes) they will want to you demonstrate that you have previous experience lecturing to at least 100 students or more.
For a TA position, preference will probably be given to current grad students but it wouldn’t hurt to apply.
You’ll also notice the rate of pay is rather low, particularly for the amount of work you’ll be required to do. If you’re doing this for monetary reasons, I would agree with what Bambi is saying - focus on medical school. But maybe you are interested in teaching experience?